Image sensing device, process for driving such a device and electrical signal generated in such a device

ABSTRACT

An image sensing device has an image cell with a photo-sensitive element, a detection node and an output pin. Charges accumulated during a first period of time are transferred from the photo-sensitive element to the detection node, thereby obtaining a first voltage, and kept on the detection node which is thus used as a memory. Some moments later, a first value corresponding to the first voltage is generated on the output pin, charges accumulated during a second period of time are transferred from the photo-sensitive element to the detection node, thereby obtaining a second voltage, and a second value is generated on the output pin corresponding to the second voltage. The signal thus generated is also proposed.

The invention relates to an image sensing device, to a process fordriving such a device and to a signal generated in such a device.

In recent years, electronic imaging devices have been more and morewidely used. In these devices, light-sensitive elements arranged as amatrix convert impinging light (photons) received during an integrationtime into a corresponding charge. The charge output by each element; isthen used as an indicator of the light received by this element, andtherefore as a measure of the light received from a given direction.

The sensitivity of the sensing device naturally depends on theintegration time. To increase the dynamic range of the sensing device,it has already been proposed to use subsequently two differentsensitivities (i.e. two different integration times) and to combine thetwo measurements into a single pixel value, as described for instance inWO 02/052 842.

Patent references EP 0 387 817, U.S. Pat. Nos. 5,309,243, 5,671,013 andJP 2001-346 096 describe solutions where each of the two pictures isstored in a memory external to the image sensor. The two pictures arethen combined by appropriate processing into a picture with increaseddynamic range.

To avoid storing complete images in external memories, patentapplication EP 1 003 329 proposes to combine the two measurements intoan output value immediately after the measurements are made. However,this solution requires digital processing in the image sensor itself tocombine the two measurements, which makes the image sensor more complexand thus more expensive.

Patent application EP 1 233 613 describes an alternative solution inwhich each pixel element is reset only when a predetermined charge hasaccumulated. When the charge has not reached the predetermined value,the reset is skipped. For each pixel, the number of reset skips is thusused as a further indicator of the incoming light in addition to theconventional charge peak value. However, this solution needs a complexarchitecture to select the pixels which have to be reset.

In view of these problems, the invention seeks a cost-effective andsimple solution for a image sensing device with increased dynamic range.

The invention proposes a process for driving an image sensing devicehaving an image cell with a photo-sensitive element, a detection nodeand an output pin, comprising the steps of:

-   -   transferring charges accumulated during a first period of time        from the photo-sensitive element to the detection node, thereby        obtaining a first voltage;    -   generating on the output pin a first value corresponding to the        first voltage;    -   transferring charges accumulated during a second period of time        from the photo-sensitive element to the detection node, thereby        obtaining a second voltage    -   generating on the output pin a second value corresponding to the        second voltage.

The two values are available at the level of the image cell without theneed for a complex construction.

Preferably, the step of generating the first value is separated from thestep of transferring charges accumulated during the first period byapproximately the second period. It is then taken advantage of thedetection node to store the first voltage during the second period, sothat the two values can be output in a limited period of time.

In an advantageous embodiment, the step of generating the first valueoccurs more than 20 μs later (and preferably more than 1 ms later) thanthe step of transferring charges accumulated during the first period andthe step of generating the second value occurs less than 20 μs later(and preferably less than 100 μs) than the step of generating the firstvalue.

Said differently, when the image sensing device generates a video signalcomprising lines separated by horizontal blanking intervals, the step ofgenerating the first value occurs at least one line time minus ablanking interval time later than the step of transferring chargesaccumulated during the first period, whereas the step of generating thesecond value occurs less than the blanking interval time later than thestep of generating the first value.

According to a preferred solution, when the image sensing devicegenerates a video signal comprising horizontal blanking intervals, thestep of generating the first value and generating the second value occurduring a single horizontal blanking interval. This fully takes advantageof the detection node to store the first voltage while outputting thefirst and second value in the short horizontal blanking interval.

In a possible embodiment, the following steps are also realised in theimage sensing device:

-   -   comparing the first value to a reference value    -   if the first value is above the reference value, generating an        output value based on the first value;    -   if the first value is below the reference value, generating an        output value based on the second value.

As an image sensing device generally has more than one image cell, theinvention results in a process for driving an image sensing devicehaving a first image cell with a first photo-sensitive element, a firstdetection node and a first output pin, and a second image cell with asecond photo-sensitive element, a second detection node and a secondoutput pin, the first output pin and the second output pin beingconnected to a common output wire, comprising the steps of:

-   -   transferring charges accumulated during a first time-length from        the first photo-sensitive element to the first detection node,        thereby obtaining a first voltage;    -   transferring charges accumulated during the first time-length        from the second photo-sensitive element to the second detection        node, thereby obtaining a second voltage    -   generating on the first output pin a first value corresponding        to the first voltage;    -   transferring charges accumulated during a second time-length        from the first photo-sensitive element to the first detection        node, thereby obtaining a third voltage;    -   generating on the first output pin a second value corresponding        to the third voltage;    -   generating on the second output pin a third value corresponding        to the second voltage.

The values for a given image cell are grouped together thanks notably tothe use of the detection nodes to store voltages.

Generally, this process further comprises the steps of:

-   -   transferring charges accumulated during the second time-length        from the second photo-sensitive element to the second detection        node, thereby obtaining a fourth voltage;    -   generating on the second output pin a fourth value corresponding        to the fourth voltage.

The invention consequently proposes an electrical signal generated in animage sensing device having a first image cell and a second image cell,taking over increasing time:

-   -   a first value representing light received by the first image        cell during a first time-length;    -   a second value representing light received by the first image        cell during a second time-length;    -   a third value representing light received by the second image        cell during the first time-length;    -   a fourth value representing light received by the second image        cell during the second time-length.

The image sensing device has an image cell with a photo-sensitiveelement, a detection node and an output pin, and comprises transfermeans for transferring charges from the photo-sensitive element to thedetection node, read-out means for generating on the output pin a valuebased on the detection node voltage and driving means for controllingthe transfer means and the read-out means so that at least two valuesare generated during an integration period.

In a possible embodiment, the image sensing device comprises comparatormeans for comparing at least one of said two values to a referencevalue.

It is to be noted that the steps and parts of signal are ordered asindicated but that they may be separated by other steps, such as stepsto deal with the reset noise, as described below.

Other features of the invention will appear in the following descriptionwhich refers to the appended drawings, where

FIGS. 1 a and 1 b depict basic elements of a CMOS imager according tothe invention;

FIGS. 2 a to 2 c show the timing of the driving voltages of thetransistors of an image cell according to the invention;

FIGS. 3 a to 3 e describe operation when a pixel receives a low tomedium quantity of light;

FIGS. 4 a to 4 e describe operation when a pixel receives a highquantity of light;

FIG. 5 illustrates a possible signal output from an imager according toa possible variation of the invention.

It should be noted that the various illustrations are not in scale inorder to allow a clear presentation of the invention.

Some basic elements of a CMOS imager are represented on FIGS. 1 a and 1b. An imager comprises a matrix of pixels or image cells.

FIG. 1 a shows in detail an image cell IC_(n). The image cell IC_(n)comprises a photodiode which is the light-sensitive element of the imagecell: the photodiode ΦD creates an amount of charges depending on thequantity of light it receives.

The photodiode ΦD is connected to a first plate of a capacitor C througha CMOS transistor T1 (transfer gate). The second plate of capacitor C isgrounded. The first plate of capacitor C is also connected to ground viaa CMOS transistor T3 (reset transistor) which can therefore reset (i.e.put to a reference value the voltage across) capacitor C.

Lastly, the first plate of capacitor C is connected to an output pin ofthe image cell IC_(n) via CMOS transistors T2 (buffer transistor) and T4(read-out or row address transistor), which allows to send out the valuemeasured by the image cell IC_(n). (For this description, row has thesame meaning as line.)

The first plate of capacitor C thus realises a detection node.

The transfer gate T1, the reset transistor T3 and the read-outtransistor T4 are controlled by driving signals (respectively DS1, DS3and DS4) as represented respectively on FIGS. 2 a to 2 c and furtherdescribed later. Buffer transistor T2 is biased by a voltage V+ (whichcan be either a constant voltage or active only when it is needed).

FIG. 1 b represents three image cells IC₁, IC₂ and IC₃ with theirrespective output connected to a read-out column ROC. Each image cellIC_(n) of a column of the pixel matrix is similarly connected to acorresponding read-out column ROC. Each read-out column ROC transmitsthe measured values from the image cells of a column to a read-outcircuit RC_(m) dedicated to process the values of the column. Eachcolumn thus has a dedicated read-out circuit RC_(m).

As represented on FIG. 1 b, a read-out circuit RCm comprises acomparator COMP having a first input at a reference voltage V_(comp) anda second input connected to the read-out column ROC. The result of thecomparison is used as an extended dynamic range (EDR) signal as will befurther described below.

The read-out column ROC carrying the various measured values, it is alsoconnected to an output pin labelled POV (for Pixel Output Value) througha multiplier (by 0, +1 or −1 depending on the considered moment asfurther explained below) and an (integrating) amplifier.

The way of operation of the image sensing device according to theinvention will now be explained in greater detail by the discussion oftwo exemplary cases.

FIGS. 3 a to 3 e illustrate how the image sensing device operates in afirst possible case where the pixel typically receives a low to mediumquantity of light.

FIG. 3 a represents the charges accumulated by the light sensitiveelement at a pixel over an integration period I. The integration periodI is preferably in the range of several milliseconds; for instance, I=20ms. Charges accumulate from a starting point t₀ of the integrationperiod, during a first period I₁ ending at t₁, where I₁=t₀−t₁=I/N.Preferably, N is such that I¹, is an integer multiple of a line-time(T_(line)), for instance, I₁=5 T_(line). (For PAL, I=312.5 T_(line), sothat N=62.5.)

On the other hand, at t₀, the voltage at the detection node (illustratedon FIG. 3 b) is reset by closing reset transistor T3 shortly (by drivingsignal DS3, see FIG. 2 b) and the detection node thus carries a resetnoise voltage R₁.

At t₁, the light sensitive element has accumulated an amount Q₁ ofcharges and the transfer gate T1 is shortly closed (by driving signalDS1, see FIG. 2 a) in order to transfer the charges Q1 from the lightsensitive element (photodiode ΦD) to the detection node (capacitor C),thereby rising the voltage at the detection node by a value V₁. Thedetection node has then the value R₁+V₁.

After t1, the light sensitive element accumulates charges again, over asecond period I₂, preferably longer than I₁. According to a convenientsolution, I₂=I−I₁=I. (N−1)/N.

Before the end of the second period, and preferably immediately beforethe end of the second period, at a moment t₂, read-out transistor T4 isclosed (see FIG. 2 c) to output the voltage at the detection node(R₁+V₁) to the read-out column ROC, where it is compared by thecomparator to a reference value V_(comp). The read-out values V_(ROC)are represented on FIG. 3 c.

Preferably, the reference voltage is given by the formula:V _(comp) =V _(max)/(N−1),

-   -   where V_(max) is the clipping value corresponding to Q_(max).

In the present example, the voltage from the detection node (R₁+V₁) islower than the reference voltage V_(comp), which means that assumptioncan be made that the pixel does not clip over the second (longer) periodI₂. (The second example made with reference to FIGS. 4 a to 4 edescribes operation when R1+V1>V_(comp).)

The charges accumulated during the second period I₂ can thus be used asa basis for the value to be output for this pixel.

More precisely, immediately after the voltage (R₁+V₁) at the detectionnode has been transferred to the read-out column ROC, the voltage at thedetection node is reset by shortly closing the reset transistor T3(moment t₃, see FIG. 2 b) and thus carries a reset noise voltage R₂,which is read-out immediately at the read-out column ROC and integratedat the column-amplifier with a negative sign (see FIG. 3 d). Then, atmoment t₄, the transfer gate T1 is closed (see FIG. 2 a) in order totransfer the charges Q₂ accumulated by the light sensitive elementduring the second period to the detection node. The-voltage at thedetection node therefore increases by a value V₂ corresponding tocharges Q₂. This ends the second period: I₂=t₄−t₁ and preferably theintegration period: I=t₄−t₀. At this moment, the voltage at thedetection node is R₂+V₂.

Preferably, the steps taken from t₂ to t₄ (and even from t₂ to t₅—seedefinition of t₅ below) occur during the horizontal blanking interval HB(immediately before the line comprising the image cell is output in thevideo signal). As these steps may create noise, this avoids thatdisturbances appear on active parts of the video signal. The duration(t₄−t₂) is preferably within the range of several micro-seconds, forinstance between 5 and 10 μs.

By reading out the detection node voltage R₂+V₂ and summing it up at thecolumn-amplifier with a positive sign between t₄ and t₅, the amplifierhas integrated a value representing the pixel output value POV:−R₂+(R₂+V₂)=V₂. The output value is thus the correct measured value,without the reset noise.

Besides, as the (R₁+V₁)<V_(comp), the output of the comparator sends outa low Extended Dynamic Range signal (FIG. 3 e) to indicate to furtherprocessing entities that the output value V₂ was measured during theperiod I₂.

Advantageously, N is large such that I₂=I.(N−1)/N is close to I and thevalue V₂ can be roughly considered as taken over the whole integrationperiod I. Anyway, the value V₂ can also be multiplied by N/(N−1) to beprecisely equal to the value it would have had if it had been integratedover the whole Integration period I.

FIGS. 4 a to 4 e illustrate how the image sensing device operates in asecond possible case where the pixel typically receives a high quantityof light.

FIG. 4 a represents the charges accumulated by the light sensitiveelement at the pixel over the integration period I. As previously,charges accumulate from the starting point to of the integration period,during the first period I₁ ending at t₁. As explained above, at t₀, thevoltage at the detection node (illustrated on FIG. 4 b) is reset byclosing reset gate T3 and the detection node thus carries a reset noisevoltage R₁.

At t₁, the light sensitive element has accumulated an amount Q₁ ofcharges and the transfer gate T1 is closed in order to transfer thecharges Q1 from the light sensitive element (photodiode (ΦD) to thedetection node (capacitor C), thereby rising the voltage at thedetection node by the value V₁. The detection node has then the valueR₁+V₁.

After t1, the light sensitive element accumulates charges again, overthe second period I₂, preferably longer than I₁. In this second example,the maximum amount of charges Q_(max) acceptable by the light-sensitiveelement is reached before the end of the integration period (i.e. alsobefore the end of the second period I₂): the pixel clips. This has theconsequence that the amount of charges and the derived voltage will notaccurately represent the amount of light received by the pixel over thesecond period I₂.

At moment t₂, read-out transistor T4 is closed to output the voltage atthe detection node (R₁+V₁) to the read-out column, where it is comparedby the comparator to the reference value V_(comp). The read-out valuesare represented on FIG. 4 c.

In this second example, the voltage from the detection node (R₁+V₁) islarger than the reference voltage V_(comp). This is indicative that thepixel clips over the second period I₂ for the following reasons:

-   -   R₁ is small compared to V₁ (R1 is noise)    -   the amount of light being considered constant over the        integration period (which is a basic assumption in such sensing        devices),        V ₁ >V _(comp) =V _(max)/(N−1) over I1    -   implies the voltage will reach V_(max) before the end of        I₂=(N−1).I₁.

As the pixel clips when integrated over I₂, the value (R₁+V₁) measuredover the (shorter) first period I₁ is more representative of thequantity of light received by the light sensitive element and will beused to compute the pixel output value (POV) out of the column amplifier(FIG. 4 d).

As (R₁+V₁)>V_(comp), the output of the comparator sends out a highExtended Dynamic Range signal (see FIG. 4 e) to indicate to furtherprocessing entities that the output value V₁ was measured during theperiod I₁. To retrieve the amount of light which has been received overintegration time I, the pixel output value shall be multiplied by N−1 byfurther processing steps (or by N if the value is corrected by a factorN/(N−1) when the Extended Dynamic Range is low—see above).

The voltage (R₁+V₁) read-out at moment t₂, besides being used on thecomparator input, is in this case integrated at the column-amplifierwith a positive sign. Immediately afterwards, the voltage at thedetection node is reset by shortly closing the reset transistor T3(moment t₃) and thus carries a reset noise voltage R2, which is read-outimmediately at the read-out column and integrated at thecolumn-amplifier with a negative sign (see FIG. 4 d).

The amplifier is then ready to send out the pixel output value:POV=−R2+(R1+V1)≈V1.

The output value is thus a very close measure for the correct measuredvalue as R2 and R2 are only noise values.

Finally, the clipped value (R₂+V₂) is read-out to the read-out columnROC by closing read-out gates T2 and T4 but it is not integrated at thecolumn amplifier because of the result of the comparison realised by thecomparator: (R₁+V₁)>V_(comp).

It is important to point out that in any case (case of FIGS. 3 a to 3 eand case of FIGS. 4 a to 4 e) the values for the other pixels (or imagecells) of the column are read-out on the read-out column ROC betweentime t₀ and time t₂. During this period of time, the detection node(capacitor C) is used as a memory to store the first measurement (R₁+V₁)made in the image cell. In this way, the first measurement (R₁+V₁), thereset value R₂ and the second measurement (R₂+V₂) for a given pixel canbe transmitted successively on the read-out column, which greatlysimplifies further processing of the signal.

These advantages can also be found in a possible alternative embodimentwhere the read-out circuit RC_(m) does not select between the first andsecond measured values but transmits them both to a further processingstage together with the extended dynamic range (EDR) signal. In thisembodiment, the output values thus correspond to the voltages carried bythe read-out column ROC, as shown on FIG. 5.

In the example of FIG. 5, image cells IC₁ and IC₃ clips over the secondperiod I₂; correspondingly, the EDR signal is set to a high level H,indicating to further processing stages that the first measured value,respectively (R₁+V₁)_((IC1)) and (R₁+V₁)_((IC3)), should preferably beused. At the opposite, image cell IC₂ does not clip and the EDR signalis low (level L) for this image cell indicating that (R₂+V₂)_((IC2))should preferably be used.

Of course, in this alternative embodiment, further processing stagestake care for the selection between the two measured value and thepossible correction thanks to the reset value (R₂)_((ICn)):

-   -   if EDR is high (H) for a pixel IC_(n) , the value to be used for        this pixel is:        (R₁+V₁)_((ICn))−(R₂)_((ICn)),

to be multiplied by (N−1) to be on the same scale as values integratedover the second period I₂;

-   -   if EDR is low (L) for a pixel IC_(n) , the value to be used for        this pixel is:        (R₂+V₂)_((IC) _(n))−(R₂)_((ICn)).

The invention is not limited to the examples given above. For instance,although the above examples use a CMOS imager, the invention alsoapplies to other types of imagers, such as a CCD imager.

Similarly, although the invention has been explained with alight-sensitive element generating positive charges, it also applies toimagers with light-sensitive elements generating negative-charges.

1. A method for driving an image sensing device having an image cellwith a photo-sensitive element, a detection node and an output pin,wherein a reference signal and, via an output line, a value present atthe output pin, are applied to a comparator, the method comprising thesteps of: transferring charges accumulated during a first period of timefrom the photo-sensitive element to the detection node, therebyobtaining a first voltage; generating a first value corresponding to thefirst voltage; comparing the first value with the reference signal anddetermining a signal representing the result of the comparing;transferring charges accumulated during a second period of time from thephoto-sensitive element to the detection node, thereby obtaining asecond voltage; generating a second value corresponding to the secondvoltage; and selectively applying one of the first value or the secondvalue to a single buffer, depending on a value of the signal resultingfrom the comparing; wherein the first and the second periods of time aresubsequent to each other and are nested within a frame or field period,wherein the first period of time is shorter than the second period oftime, and wherein the second period of time ends on or after resellingthe detection node after reading out the first value.
 2. The methodaccording to claim 1, wherein the step of generating the first value isseparated from the step of transferring charges accumulated during thefirst period by approximately the second period.
 3. The method accordingto claim 1, wherein the step of generating the first value occurs morethan 20 μs later than the step of transferring charges accumulatedduring the first period and wherein the step of generating the secondvalue occurs less than 20 μs later than the step of generating the firstvalue.
 4. The method according to claim 3, wherein the step ofgenerating the first value occurs more than 1 ms later than the step oftransferring charges accumulated during the first period and wherein thestep of generating the second value occurs less than 100 μs later thanthe step of generating the first value.
 5. The method according to claim1, wherein the image sensing device generates a video signal comprisinglines separated by horizontal blanking intervals, wherein the step ofgenerating the first value occurs at least one line time minus ablanking interval time later than the step of transferring chargesaccumulated during the first period and wherein the step of generatingthe second value occurs less than the blanking interval time later thanthe step of generating the first value.
 6. The method according to claim1, wherein the image sensing device generates a video signal comprisinghorizontal blanking intervals and wherein the steps of generating thefirst value and generating the second value occur during a singlehorizontal blanking interval.
 7. The method according to claim 1,comprising the steps of: comparing the first value to a reference valueprior to reading out the second value; if the first value is above thereference value, generating an output value based on the first value; ifthe first value is below the reference value, generating an output valuebased on the second value.
 8. The method of claim 1, wherein the imagesensing device has a first and a second image cell connected to a commonoutput line, wherein the first signals of the first and the second imagecell and the second signals of the first and the second image cell areapplied to the common output line consecutively.
 9. Image sensing devicehaving an image cell with a photo-sensitive element, a detection nodeand an output pin, comprising: transfer means for transferring chargesfrom the photo-sensitive element to the detection node; read-out meansfor generating on the output pin a value based on the detection nodevoltage; driving means for controlling the transfer means and theread-out means so that at least two values are generated during anintegration period; a single buffer connected to the output pin; andcomparator means for comparing at least one of said two values to areference signal and for selectively applying the first value or thesecond value to the single buffer, depending on a value of the signalresulting from the comparing; wherein the buffer comprises anintegrating amplifier and a means for changing a sign of a value appliedto the integrating amplifier.
 10. The method of claim 1, furtherincluding the steps of: resetting the detection node prior totransferring charges accumulated during the second period of time fromthe photo-sensitive element to the detection node; and applying thesignal present in the detection node after resetting to the buffer, forsubtraction.